Conventionally, natural rubber and synthetic rubbers has been vulcanized. Vulcanization means producing bridged structure among rubber molecules by using a curing agent mixed with the crude rubber. Vulcanization increases elasticity of the rubber, giving sufficient strength for practical use. Known curing methods include sulfur cure, peroxide cure and radiation cure.
However, rubber products using the above curing methods have the following problems. When rubber products by sulfur cure are used in acid solution, zinc ion may be eluted from zinc oxide which was added during production process. Furthermore, because of some vulcanization accelerators, the rubber product itself may cause skin irritation.
If rubber products are made of rubber prepared by peroxide cure, the products themselves may cause skin irritation because peroxide has a property of irritating skin.
Rubber products made of rubber prepared by radiation cure are quite harmful to human bodies because they contain carbon tetrachloride or organic multifunctional monomers as a cross linking agent.
When the rubber products made of the rubbers vulcanized by any of the curing methods are used in direct contact with the human bodies, particularly when they are used in the human bodies, they have a fatal drawback of being harmful. Whereas unvulcanized rubbers do not have the above harmfullness because any curing agent is not added to them. However, the unvulcanized rubbers cannot achieve sufficient strength, and so a single substance of them is rarely used as a structural material.
Further improvement in the modulus of the vulcanized rubber is sometimes required. In this case, the above vulcanizations cannot give sufficient modulus to the rubber.